1. Field of the Invention
The present invention concerns an arrangement to control an antenna arrangement as well as a device for signal splitting and a transmission method, for a magnetic resonance examination to be implemented.
2. Description of the Prior Art
Presently techniques known as “transmit-sense” techniques are under discussion for use in magnetic resonance examinations of patients in order to excite nuclear spins in a narrowly limited range in a targeted manner.
This technique enables the effective integral of the radio-frequency magnetic field for examination to be directed only in a narrowly limited region of interest for the excitation of the nuclear spins, while also enabling the integral of the radio-frequency magnetic field (which integral is effective for the excitation of the nuclear spins) to be homogeneously adjusted in the examination region.
Multiple transmitters are used in this technique, each of which is connected with an antenna element of a transmission antenna arrangement. For example, the transmission antenna arrangement is fashioned in the form of a birdcage antenna. A radio-frequency signal is emitted via each transmission element in a transmission time period. In their time curve, the radio-frequency signals differ entirely from one another in terms of their voltages, frequencies and phases.
Today circularly-polarized transmission signals are preferably used for examination.
FIG. 3 shows an exemplary arrangement of a circularly-polarized transmission signal according to the prior art.
A radio-frequency transmission signal SS arrives at a radio-frequency splitter SPLIT as an amplified transmission signal VSS via a transmission power amplifier PA. The radio-frequency splitter SPLIT has two inputs In1, IN2 as well as two outputs Out1, Out2, wherein the amplified transmission signal VSS is connected to a first input In1.
The radio-frequency splitter SPLIT divides the transmission signal VSS in two, into essentially equal transmission signals SS1 and SS2, with the two transmission signals SS1 and SS2 being phase-shifted relative to one another by 90°.
A first transmission signal SS1 arrives at a first connection A1 of an antenna arrangement ANT while a second transmission signal SS2 arrives at a second connection A1 of the antenna arrangement ANT. The two connections A1, A2 form respective feed points of the antenna arrangement ANT.
The antenna arrangement ANT is fashioned to radiate a circularly-polarized transmission signal when two transmission signals (ideally phase-shifted by 90°) are fed to it via the feed points.
In this example the antenna arrangement ANT is fashioned as what is known as a birdcage resonator. The two connections A1 and A2 are arranged offset on a ferrule of the antenna ANT such that a circularly-polarized transmission signal is radiated by the transmission signals phase-shifted by 90°.
FIG. 4 shows an exemplary embodiment according to the prior art, based on FIG. 3 of the radio-frequency splitter SPLIT.
The radio-frequency splitter SPLIT has a first input In1, a second input In2, a first output Out1 and a second output Out2. The amplified transmission signal VSS is connected at the first input In1.
The first input In1 is connected via a first inductor L1 with the first output Out1 while the second input In2 is connected via a second inductor L2 with the second output Out2. It is also possible to magnetically couple the inductors L1, L2.
The first input In1 is connected via a first capacitor C1 with the second input In2 while the first output Out1 is connected via a second capacitor C2 with the second output Out2.
The second input In2 is connected via a resistor Z with a reference potential (here the ground) such that constant power components reflected in the outputs Out1, Out2 are compensated in the resistor Z.
The first output Out1 is connected with the first connection A1 of the antenna arrangement ANT while the second output Out2 is connected with a second connection A2 of the antenna arrangement ANT.
The values of the inductors L1, L2, the coil coupling and the values of the capacitors C1, C2 are selected such that the desired phase difference of 90° between the two transmission signals SS1 and SS2 results at the outputs Out1, Out2 of the frequency splitter SPLIT.
Here the first transmission signal SS1 that is fed via the first output Out1 to the first connection A1 as a first feed point at the ferrule of the antenna ANT has a phase position of 90°.
The circularly-polarized transmission signal is formed by the phase-shifted control at the aforementioned connections A1, A2.
In magnetic resonance apparatuses that are operated with magnetic field strengths of greater than or equal to three Tesla and that use circularly-polarized transmission signals, currents are induced in the tissue of the subject due to different relative permittivities (dielectric constants) and conductivities of the human tissue.
Depending on the type of the tissue and after distribution of the currents, these currents cause an amplification or reduction of the exciting radio-frequency magnetic field. These effects disadvantageously affect the examination.